Various processes for sealing the crystal to the casing of a watch have been proposed, which make use of the thermoplastic properties of crystals of certain organic materials. According to U.S. Pat. No. 2,607,082, a watch crystal made of thermoplastic material and provided with an external groove is pressed into a metal rim on a watch casing. The metal rim has a rib which projects inwardly of the watch casing to engage the groove in the plastic crystal. The assembly of rim and crystal is then placed on a carrier within coils of an induction heating apparatus. The metal rim is thus heated, which softens the plastic material of the crystal in the region of the rib on the rim, whereby the plastic material is caused to flow around the rib, producing a sealed joint. It will be readily appreciated, however, that this process cannot be used if the watch casing is itself made of a plastic material.
French Patent Application No. 2,491,644 proposes, for the purposes of sealing a crystal of organic material to a watch casing, that the rim for mounting the crystal be provided with a groove which is directed inwardly of the rim. The crystal is supported on a shoulder portion of the rim and a heated punch member of annular shape is applied to the periphery of the crystal to cause plastic deformation thereof, thereby causing a part of the plastic material to flow into the groove in the rim. While such a process makes it possible to produce a sealed joint between the crystal and the rim while avoiding the use of a mechanical joining member or any adhesive, the process produces results which are not generally satisfactory from the aesthetic point of view. In fact, the deformation which is due to the annular punch member being applied to the crystal occurs at the front face thereof. In addition, such methods of fixing the crystal in place involve not inconsiderable mechanical forces in order to cause deformation of the plastic. Such forces induce in the final article internal stresses which affect the ageing qualities of the casing to which the crystal is joined.